This invention relates generally to linear amplifiers and more specifically to a circuit for increasing the slew rate of such amplifiers.
As is known in the art, linear amplifiers are used in a variety of applications in which an input signal is desired to be amplified. That is, the amplifier circuit amplifies the input signal to provide an output signal proportional to such input signal. Or, in the case of an operational amplifier, the output signal is proportional to the difference between a pair of input signals. Ideally, the voltage level of the output signal changes instantaneously in response to changes in the voltage level of the input signal(s).
As is also known, linear amplifiers have capacitance associated therewith, such as may be internal, parasitic capacitance or capacitance which is externally added for compensation purposes (i.e. for modifying the frequency response of the amplifier to prevent oscillations). Since the voltage across a capacitor cannot change instantaneously, changes in the output signal cannot occur instantaneously in response to changes in input signal(s). The rate of change of the output signal with time is referred to as slew rate and the propagation delay refers to the delay between a change in the input signal and a corresponding change in the output signal. Both the slew rate and the propagation delay are limited by the size of the capacitance associated with the linear amplifier. Specifically, the slew rate is equal to the capacitance charging (or discharging) current divided by the value of the capacitance. The capacitance charging or discharging currents are provided by the amplifier's input, driver, or other stages. When the input signal changes slowly, these stages can charge or discharge the capacitance so that the output signal closely follows the input signal(s). Whereas, when the input signal changes quickly, the capacitor requires a relatively large amount of current for charging or discharging. In this case, the output signal will not change as fast as the input signal(s), thereby resulting in a propagation delay as well as a reduction in the slew rate.